A virtual world is a type of computer simulated environment in which a user interacts with the environment in some way, either by viewing or hearing the environment, or by changing the environment. Often the environment is independent of the user. In a city simulation game, the user is a single all-powerful character and does not interact with other users. In a social interaction environment, multiple users are represented by avatar objects and interact with each other. A user can enter and exit the environment and things in the environment can persist or change depending on user interaction or not. In a city simulation game, city things in the environment change as a factor of time and are also controlled by the user in some way. In a social interaction environment, things also change as a factor of other users changing them or interacting with them. The more complex a virtual world, the more feature rich the environment, and the more difficult it is for a user to keep up with all of the changes that have taken place.